Research Overview For the past twenty-five years, I have been actively engaged in research dealing with the coordination chemistry of macrocyclic thioethers and related ligands. My specific research interests with thiacrowns have focused on the effects that alterations within the ligand environment have on the structural, spectroscopic, and electrochemical properties of the metal complex. Our most recent efforts have largely focused on complexes with two groups of metal ions -- the platinum group metal ions (Pt, Pd, Ru, Ir, Rh,) as well as heavy metal ions (Hg, Cd, and Pb). Our prototypical ligand is the trithiacrown 9S3 (1,4,7-trithiacyclononane) whose structure is shown below. Our current research support is for incorporating our thiacrown metal complexes into two- and three-dimensional metallosupramolecular complexes formed by transition metal mediated self-assembly. We are specifically interested in using vertices based upon transition metal complexes with thiacrown ligands as subunits to direct the self-assembly of the supramolecular structure. Our aim is to incorporate the unusual properties demonstrated by mononuclear thioether complexes with platinum group metal ions into these supramolecular assemblies. We have prepared one molecular square with {Pt 9S3}2+ corners as shown below. We plan to prepare other molecular squares as well as two-dimensional structures such as hexagons and cubic three-dimensional structures.

S

SS

9S31,4,7-trithiacyclononane

Some general research areas of thiacrown coordination chemistry that we have previously been involved in are: Synthesis of new ligands. We have synthesized a variety of new cyclic thioether ligands, examined their preferred conformations, and then complexed them to a variety of transition metal ions. These ligands include variations in the size of the macrocycle, the nature of the donor atoms, and the incorporation of hydrophilic functional groups to enhance water solubility in possible biological applications. A sampling of some of these ligands include:

Space filling model of the thiacrown 10S3. Two sulfur donors are endodentate while one is exodentate. Our group reported the first synthesis of the ligand in 1989. Aldrich Chemical Company sold the compound commercially until 2003.

The mixed oxathia crown 9S2O. We are interested in how substituting an ethereal O donor for a thioether S donor affects the coordination chemistry of the ligand.

The hexadentate thiacrown 20S6. We have also studied larger tetrathia, pentathia, and hexathia macrocycles.

O

SS

S

S S

S S

S

Complexation of heavy metal ions by thiacrowns. We have been interested in exploring possible heavy metal remediation and detection using thiacrown ligands. These soft ligands are natural complexing agents for heavy metal ions like mercury. Some examples include: Hg(II): We have 199Hg NMR spectroscopy to develop correlation of Hg chemical shift within thioether coordination environments. Our group contributed a book chapter on Hg(II) thiacrown complexes for a 2006 Edition of Structure and Bonding devoted to Hg coordination chemistry. Since 2002 we have several publications on Hg(II) and heavy metal complexes. The diagram below shows the correlation between the 199Hg NMR chemical shift and the ligand environment around the Hg atom.

Cd(II): We have similarly used 113Cd NMR to develop correlations between the Cd chemical shifts within thioether coordination environments. The crystal structure of the complex [Cd(12S4)2]2+ (below) forms an unusual octa-coordinate square antiprism

Pt(II) and Pd(II) coordination chemistry Homoleptic thiacrown complexes. Our group has used 195Pt NMR to develop correlation of Pt chemical shift within thioether coordination environments, similarly to what we did with the heavy metal complexes. We have also examined fluxionality of crown thioether ligands such as 9S3 in these complexes using 13C NMR. We have also developed correlations between complex structure (presence of axial M-S interactions) and the unusual spectroscopic and electrochemical behavior that these complexes show. The HOMO for [Pt(9S3)2]2+ is shown below. The axial sulfur interacting with the Pt(II) dz

2 can clearly be seen.

Heteroleptic thiacrown complexes. These complexes have the general formula [Pt(9S3)(L)]2+ where L is some type of ancillary ligand. The ancillary ligands include chiral (BINAP) and achiral diphosphines (dppe), diimines (2,2'-bipyridine), halides, cyclometallating ligands (2-phenylpyridine), and monodentate Group 15 donor ligands (EPh3). We have examined how these different ancillary ligands affect the thiacrown complexation towards Pt(II), particularly with regards to structural, spectroscopic, and electrochemical effects.. Importantly, the Pt-S axial length in the 9S3 ligand can be changed by over 0.5Å depending upon the identity of L. We have also been interested in intra- and intermolecular π-π stacking interactions for these complexes.

Mixed Sandwich Organometallic Complexes. Our group has prepared a series of mixed sandwich complexes using Cp* and tridentate macrocycles (9S3, 9N3, and 10S3) with Rh(III) and Ir(III) and examined the structural, spectroscopic, and electrochemical properties of these complexes. The complexes have the general formula, [M(Cp*)(L)]2+ , where L = 9S3. The structure of the cation of [Ir(Cp*)(9S3)]2+ is shown below. This avenue of research builds upon our prior work with related [M(Cp)(L)]+ complexes where M = Fe2+ or Ru2+ and L = 10S3 or 9S3.

Thiacrown Complexes with First Row Transition Metals. Our early coordination chemistry with thiacrowns focused on first row transition metals (Fe - Cu), primarily in their divalent state. The ligands function as strong field ligands and generate low-spin electronic states in these metal ions. We have reported two diastereoisomers of the complex [Fe(10S3)2]2+ (Grant, G.J.; Isaac, S.M.; Setzer, W.N.; VanDerveer, D.G. Inorganic Chemistry, 1993, 32, 4294-4296). Interestingly, the gauche or cis diastereoisomer (see structure below) undergoes spontaneous resolution to form an enantiomerically pure crystal (see, Setzer, W.N.; Cacioppo, E.L.; Guo, Q.; Grant, G.J.; Kim. D.D.; Hubbard, J.L.; VanDerveer, D.G.; Inorganic Chemistry 1990, 29, 2672 - 2681)